Conventional Total Hip Replacement (THR) is a very successful procedure for the treatment of arthritis of the hip, a condition which causes considerable pain and loss of movement. As is well known, the hip is a ball and socket joint which allows the upper leg to move from side to side, back to front, and to rotate. The joint is made up of the head of the femur (the ball) which fits into the acetabulum (the socket). In a healthy hip, both the head of the femur and the acetabulum are covered with cartilage which provides a smooth surface allowing the joint to move freely.
The earliest Total Hip Replacement (THR) procedure, in 1938, involved an implant in which both surfaces were made from metal and was known as a metal-on-metal bearing. During the 1950's and early 1960's, a number of surgeons developed hip implants using this type of bearing, although many of these designs had a tendency to work loose early on as the techniques used to anchor them to the bone were not very successful. However, the implants that did not loosen early on have generally lasted well.
In the early 1960's, a British Surgeon, Sir John Charnley, developed a new type of Total Hip Replacement which is still in use today. This procedure, which is illustrated in FIG. 1, involves cutting the worn head off the femur and replacing it with a metal ball 10 and stem 12 in the shaft of the femur 14 and a plastic cup 16 in the pelvis 18. Both elements are typically anchored to the bone by “bone cement”. This has become a very common surgical procedure with some 45,000 hip replacements being carried out in the UK every year.
The plastic used to form the cup 16 is inert and so is well tolerated by the body. Nevertheless, as the metal ball 10 rubs against the plastic cup 16, tiny particles of the plastic are worn away. This plastic debris causes an irritation. Furthermore, as the particles get between the bone and the artificial joint, this irritation causes surrounding bone to be absorbed by the body, leading to a loosening of the artificial joint. In older people, with a lower activity level, this may not happen for twenty or more years, but in younger, more active patients, this may happen much sooner.
To overcome these problems in younger, more active patients requiring hip replacement, a different type of implant was needed. In 1991 a procedure for metal-on-metal (MoM) resurfacing of the hip was proposed. This has two major differences from a conventional THR. The first is that both components are made from metal, typically Cobalt Chrome. By eliminating the plastic cup of a conventional THR, and making both parts of the bearing surface of metal, the resurfaced hip is expected to last much longer and therefore to be more suitable for higher demanding patients. The second difference is that the procedure is very bone conserving, since the head of the femur is simply reshaped and “resurfaced”, rather than removed. Accordingly, should the device need replacing at some time in the future, this may provide better options for the surgeon at that time as a conventional THR can then be used.
A typical Resurfacing Hip is shown in FIG. 2 to comprise a femoral head component 20 and an acetabular cup 22. During the pre-operative planning stage, an X-ray of the hip is templated to assess the probable sizes for both the femoral head component 20 and acetabular cup 22. Alignment of the femoral head component 20 is also determined pre-operatively and is an important part of the templating procedure. The femoral head component 20 should be positioned in neutral or slight valgus alignment. Varus positioning should be avoided. Once the template has been satisfactorily positioned with respect to the X-ray, the distance is measured from the tip of the greater trochanter 24 to the point where the axis 26 of the femoral neck 32 crosses the lateral cortex of the femoral shaft 14. This pre-operative measurement is recorded as it indicates the position for subsequent insertion of a lateral positioning pin 28 of a head centering jig 30 (see FIG. 3 and related description below).
Initial preparation of the femur may be carried out to one size larger than that templated in order to ensure that there is sufficient clearance around the femoral neck 32 for the definitive implant. Further preparation to the templated size may then be carried out if it is evident that there is sufficient clearance around the neck 32 and that the integrity of the neck will not be compromised. Sufficient clearance may occasionally still remain to allow a smaller size to be used than was suggested by templating. In this case, further preparation to the smaller size may be carried out, ensuring the use of the most appropriate size of femoral components and minimizing the amount of bone removed from the acetabulum 34.
During hip resurfacing procedures it is common to prepare the femur first as this will debulk the femoral head 36 and facilitate access to the acetabulum 34. As part of this process the mid-lateral cortex of the femur is exposed and the position of the lateral positioning pin 28 is determined based on the pre-operative measurement taken from the X-ray. The lateral positioning pin 28 is then drilled into the mid-lateral cortex of the femur, initially using a lateral approach, but angling the pin towards the femoral head 36 once the outer cortex has been penetrated, as shown in FIG. 3.
Before proceeding with head centering, the size of the femoral head component 20 can be confirmed by placing an appropriate head template around the femoral neck 32. What is important to assess is that the appropriate clearance is available and that the femoral neck 32 will not be notched during preparation of the femur since this will result in a potential post-operative weakening of the hip.
In the past, once the size of the femoral head component 20 had been confirmed, a jig 30, such as that shown in FIG. 4, would be used to locate the centre of the femoral head 36. Since the femoral head 36 does not define a uniform sphere, what is important is to identify that point on the femoral head which coincides with the central axis 26 of the femoral neck 32.
As shown, the head centering jig 30 of the prior art comprises a hollow guide tube 38 having a proximal end 40 and a distal end 42. The guide tube 38 is supported by an arm 44 which locates at one end over the lateral positioning pin 28 and is provided at the other with a pawl 46 for selective engagement with a rack 48 provided on an outer surface of the guide tube. Thus, by the selective engagement and disengagement of the ratchet mechanism defined by the pawl 46 and rack 48 the guide tube 38 may be progressively advanced with respect to the arm 44. A locking screw 50 is provided to retain the guide tube 38 in fixed relation to the arm 44 once the desired relative position has been established.
A sleeve 52 is rotatably mounted to the guide tube 38 at a position intermediate the distal end 42 and the rack 48. As well as rotating, the sleeve 52 is also able to slide along the guide tube 38, thereby enabling the sleeve to be positioned at a range of distances from the distal end 42. A further locking screw 54 is provided to enable the sleeve 52 to be clamped longitudinally with respect to the guide tube 38 while still permitting a portion of the sleeve to rotate. To this rotating portion there is attached a projection 56 which extends in a direction perpendicular to a longitudinal axis of the guide tube 38. A stylus 58 is slidably mounted on the projection 56 to which it may be clamped in a selected one of a plurality of predetermined positions by means of a third locking screw 60. These predetermined positions correspond to different sizes of femoral head component 20 and are marked on the projection 56 as a series of graduations.
In order to identify the centre of the femoral head 36 the stylus 58 is set to the confirmed size of the femoral head component 20 and the third locking screw 60 is tightened to clamp the stylus with respect to the projection 56. The aim 44 of the head centering jig 30 is then located over the lateral positioning pin 28 and the guide tube 38 advanced towards the femoral head 36 in a controlled manner by means of the ratchet mechanism defined by the pawl 46 and rack 48. Centralization of the guide tube 38 is achieved by rotating the stylus 58 around the femoral neck 32. It is critical that a tip of the stylus 58 rotates freely around the femoral neck 32 at the head/neck junction without impingement. In this way it is possible to avoid subsequent notching of the femoral neck 32. In order to assess more easily whether the guide tube 38 is accurately centered with respect to the femoral head 36, the sleeve 52 may be slid along the guide tube to a point where the tip of the stylus 58 coincides with the junction of the femoral neck 32 and femoral head 36. The further locking screw 54 may then be tightened to retain the sleeve 52 in this longitudinal position with respect to the guide tube 38.
Once the guide tube 38 has been properly centered it can be locked into position by gently impacting into the femoral head 36 a plurality of circumferential teeth (not shown) provided on the distal end 42. A long pin 62 is then drilled through the guide tube 38 into the femoral head 36 before the head centering jig 30 is then disassembled and removed. If required, once the guide tube 38 has been disassembled from the arm 44, the guide tube and stylus 58 can be positioned over the long pin 62 to carry out a final check that the long pin is correctly positioned in relation to the femoral neck 32.
Thereafter, as shown in FIG. 5, a cannulated drill 64 is advanced over the long pin 62 as far as an appropriate line for the size of the femoral head component 20 being used. Both the cannulated drill 64 and the long pin 62 are then removed and a guide rod 66 inserted in their place.
As shown in FIG. 6, an appropriate size of head cutter 68 is advanced over the guide rod 66 as far as the junction between the femoral head 36 and femoral neck 32, ensuring that the femoral neck is not notched. During this process, the head template can be positioned around the femoral neck 32 to protect the neck and trochanter 24, while swabs can be used to prevent bone debris entering the soft tissue. If necessary, a larger size of head cutter 68 may be used for initial preparation of the femoral head 36 prior to final preparation with the definitive size in order to debulk the head.
Having removed the head cutter 68, the guide rod 66 is also then removed and a top head guide 70 placed over the prepared head surface and advanced to the head/neck junction. As shown in FIG. 7, a locking screw 72 is provided to hold the top head guide 70 in place while the top of the femoral head is resected. If necessary, a short pin (not shown) can be impacted through a hole in the top head guide 70 if additional security is required during the head resection. Following resection the guide rod 66 is reinserted and a top head cutter 74 advanced along the guide rod to create a flat surface perpendicular to the neck axis 26. As a result, the femoral head 36 acquires the shape shown in FIG. 8. An appropriate size of head chamfer cutter 76 is then used to create a bevel as shown in FIG. 9. Having thus shaped the femoral head 36, a corresponding size of head template 78 is used to check the prepared shape as shown in FIG. 10 and to make a mark on the head/neck junction to indicate how far the femoral head component 20 should be advanced if fully seated. Cement keyholes can be drilled into the femoral head 36 at this stage if required.
Having prepared the acetabulum 34 and fitted the acetabular cup 22, the femoral head component 20 is similarly implanted to the prepared femoral head 36. If a cemented head is used, low viscosity bone cement is poured into the femoral head component 20 up to a line at the top of a chamfer on the inside of the implant. The definitive component 20 is then placed onto the femoral head 36 and impacted into position using a head impactor 80 as shown in FIG. 11. Any excess bone cement may then be removed. Low viscosity cement is used in preference to high viscosity cement since high viscosity cement may prevent full seating of the femoral head component 20. However, cementless head components are also available and these are simply impacted into position using the head impactor 80.
With both components now fitted, the lateral positioning pin 28 is removed and the hip reduced while at the same time avoiding scratching the femoral head component 20 against the rim of the acetabular cup 22. A full check is then made to ensure that there is no impingement and that the range of movement and stability are satisfactory.
Thus, it can be seen that accurately determining the centre of the femoral head 36 is a critical step in a hip resurfacing procedure since it determines not only the position of the long pin 62 and the guide rod 66 but also the relative positions of all the cutters and guides that are subsequently used to shape the femur. There are, however, a number of problems with the prior art head centering jig shown in FIG. 3.
Firstly, the prior art jig 30 requires the use of a lateral positioning pin 28 which must be correctly positioned with respect to the femoral neck 32. This not only requires pre-operative planning but also the intra-operative exposure of the mid-lateral cortex of the femur. Furthermore, having been inserted, it becomes necessary to remember to remove the lateral positioning pin 28 from the femoral cortex before the wound is closed. Although this might seem obvious, experience has taught that it is a sensible precaution to attach a chain 82 or other reminder to the lateral positioning pin 28 to ensure that its removal is not overlooked.
Another problem with the prior art head centering jig 30 is that it is bulky since the arm 44 must extend from the lateral positioning pin 28 at the mid-lateral femoral cortex to the femoral head 36 while still allowing the stylus 58 to rotate about the guide tube 38.
The prior art head centering jig 30 also requires the use of two hands, one to hold the guide tube 38 and the other to rotate the stylus 58.
It would therefore be advantageous to provide an improved head centering jig which was less bulky and could be operated with only one hand. It would also be advantageous if the use of the head centering jig no longer necessitated exposure of the mid-lateral cortex of the femur.